There is understood by HEMT a high electron mobility transistor, by MODFET a modulation-doped field effect transistor, by MESFET a metal semiconductor field effect transistor, by δ-FET a MESFET, in the case of which a doping pulse with a high concentration has been placed near the surface, by SIT a static induction transistor and by HBT a heterostructure bipolar transistor (heterojunction bipolar transistor).
An ion-sensitive semiconductor sensor of this type is shown for example in DE 100 62 044 A1. This sensor has a field effect transistor with a heterostructure comprising group III nitrides which is configured as a HEMT structure. The uppermost layer of the layer sequence is in direct contact with the medium to be measured during the measurement. Alternatively, a functional layer can be applied on the uppermost layer of the layer sequence, said functional layer selectively allowing through the specific ions to be determined. In every case, the ions which have been allowed through strike the gate surface or GaN surface of the layer sequence and immediately initiate the sensor effect there. There are proposed as functional layers of this type, in particular ion exchangers, such as e.g. zeolites.
In the case of pH sensors, it is proposed to apply a conducting ceramic which is selectively permeable for ionised hydrogen atoms onto the uppermost layer of the components. Suitable materials for this purpose are materials known by the name ormocers.
In fact, in principle a III-V semiconductor, such as GaN or AlGaN, is exceptionally suitable because of its polarity and surface charges for achieving a pH sensitivity relative to solutions or other media. It is disadvantageous with these pH sensors, as described also above for the state of the art, that the signals are not however reproducible and in particular a pH sensor of this type is not chemically resistant.